This is a saying you might remember from a biology class and the idea, while imperfect, is that it takes more energy to make eggs and females most typically expend more energy in parental care than do males. In the animal world, calories - not money - are the currency. Females expend a lot of energy whereas males, it is thought, can expend very little. There are a number of exceptions to this concept, particularly in the fish world.
Biologists might want to stop reading here as I plan to take a few short cuts to save from getting too far into the weeds. It is a topic for a book, not a blog post, so I will take a few short cuts but will keep the generalities as true as I can.
A bit about us first...
I always find it instructive to put these sorts of things in human terms, after all, we humans are often rather more interested in our species than others. For us humans, one of the most fascinating things is that women are born with all the eggs that they will ever have. Or to put it another way, the egg that created you and gave you half of your DNA was in your mother when she was developing inside your your grandmother's womb. Crazy to think about. A woman might release 400 eggs in her lifetime whereas the number of sperm cells that a man will produce is nearly incalculable. And it takes nearly 200,000 sperm cells to weigh as much as a single egg cell and the average male ejaculate contains about 1.8 million sperm cells.
And here is where the idea of 'eggs are expensive and sperm is cheap' really hits home. A human female hosts a developing human for nine months which grow from a tiny fertilized egg into, on average, a seven-and-a-half pound baby. I resisted calling us a parasite but the energy drain on the mother is immense. Not so much for the father. And this is where the idea of eggs are expense begins but it does not end there as we have parental care until adulthood - which is generally a little different from other animals where adulthood means reproductive viability.
Why Parental Care?
Quite simply, parental care increases the likelihood that an offspring survives past its most vulnerable ages. For some animals - like us - parental care is long and involved. In bass and Bluegill, parental care means guarding the nest and its fry until they have a somewhat reasonable chance of survival but not until adulthood. Deer are another species whose parental care we are familiar with. The females will stay with their fawns until it is time to give birth to the next fawn and then the now one year old offspring get rather unceremoniously "run off". But she has kept them safe and taught them a lot about their world. And in the case of deer, where life expectancy is about 4.5 years (more for does, less for bucks), parental care is a pretty significant amount of their lifespan and females are involved in parental care for almost all of their lives.
In biological terms, fitness is a measure of how successfully genetic material of a group or individual is passed on to future generations. Typically measures of fitness put a premium on not only producing offspring that survive but also that those offspring survive and reproduce. In evolutionary terms, fitness is about passing on your genetic material so producing a bunch of offspring is not enough to be successful. A recent post on r- and K-selection starts to get at this idea. At opposite ends of the spectrum is r-selection where a huge number of offspring (or potential offspring in spores and seeds) are produced and K-selection where few offspring - as few as one - are produced but those offspring (or potential offspring) are provided with a "foot up" in life - parental care. Both of these strategies can ensure high fitness - it just depends upon the environment in which these species exist and other life history characteristics (age at maturity, lifespan, etc.).
The downside of parental care is that it requires a lot of time and energy but there are tradeoffs in all life history characteristics.
Why Females are the Choosy Sex
While there are some exceptions, males tend to have colorful displays (dances, calls, etc.) and ornamentation (horns, pigmentation, etc.) that are energy expensive and females choose from their prospective suitors. Part of why this system has evolved is that females are going to expend a lot more energy once mating has occurred. In mammals, they carry the fetus(es). In birds, amphibians, reptiles, and fishes they produce the eggs which are high in energy to support the developing offspring. This uneven energy expenditure means that females need to be more selective in the males they are going to mate with whereas males best evolutionary play is to spread their seed as widely as possible. Female choice is what prevents that from happening.
What are females looking for? Mostly signs that the male is worth investing their time and energy. Take Northern Cardinals, a bird we are all familiar with. A male cardinal that is big with bright red and dark black feathers is demonstrating his fitness - his genetic superiority. He is "saying" that he can get food, resist parasites, and expend energy to make otherwise detrimental ornamentation. Female choice protects their investment.
If you do any casual bird watching, you know that bright pigmentation in birds is "expensive" because males of most species lose their breeding plumage during non-breeding times. Additionally, the bright coloration typical of males makes them stand out much more compared to their generally drab female counterparts which increases their visibility to the females but also to predators. Across the birds, there is a huge range of variability - from strongly sexually dichromatic as in Northern Cardinals to species like Canada Geese where males and females are similar in pigmentation. Essentially, it comes down to a mater of survival vs. mating. In some habitats, being colorful reduces survivorship beyond the point where that pigmentation is sustainable for the species. Of course some species have found ways to hide their ornamentation such as in the prairie grouses (Greater and Lesser Prairie Chickens, Sharp-tailed Grouse).
Variation in the Animal World
We are just one example of animals (yes, we are animals) but there is a great amount of variation among species. Blue Whales have a 1 year gestation period and are born at about 6,000 pounds and upwards of 20 to 25 feet in length. The average gestation period of an African Elephant is about 22 months. Marsupials famously give birth to underdeveloped offspring that they then nurse them in a pouch until they are more fully developed. This is thought to allow them to avoid putting a lot of energy and water consumption into a developing offspring so if conditions get too hot and dry, they have expended less energy when they stop putting energy into that offspring's development. The animal world can be pretty cruel but the adaptation evolved because it allowed for the survival of these species. In the animal world, it all comes back to fitness.
Birds probably provide the most readily visible example of parental care that we commonly witness (parental care in birds). Birds tend to be pretty egalitarian, that is both males and females provide parental care. In fact, about 85% of bird species have bi-parental care meaning that both male and female contribute to parental care and this is generally fairly even among the parents. About 1% of bird species have paternal care (father only) which tends to be very uncommon in the animal world. Maternal care (mother only) is much more common in the animal world, particularly among mammals where 95% of mammals have maternal care.
What about Fishes?
External fertilization in (most) fishes makes them quite different. For one, differences in energy expenditure between the sexes is much less lopsided. Yes, it takes more energy to produce eggs than milt but once her eggs are expelled, she washes her hands - or fins - of her responsibilities. In fact, in fishes, male parental care is more common than female parental care. A recent meta-analysis - an analysis of data combined from a number of studies - found that the act of parental care is what attracts females to choose those males (Goldberg et al. 2020). Or to put it another way, males that are actively guarding a nest are chosen by females because females are attracted to their parental care. A bit of a catch-22 for the males.
Displays and or ornamentation are common in the fish world as well. Males tend to be more colorful pigmentation, develop secondary sexual characteristics like trout and salmon kypes and nuptial tubercles like in Hornyhead Chubs, build nests to attract females, or have ornamentation with their fins. Among the species in our area that are most conspicuous in their coloration are the darters (family Percidae). Pigmentation in fishes provides the same tradeoffs between mating and survival. One of the best stories of evolution and the tradeoffs in pigmentation are from Endler's Guppies. The same species of guppies developed different pigmentation patterns above and below waterfalls. They were more camouflaged below the waterfall where predators were present and had more and brighter pigmentation above the waterfall where predators were absent.
Parental care in fishes includes some quite unique adaptations like mouth brooding in a number of species (there are some mouth brooding frogs as well - they also have external fertilization), some species which have internal fertilization (some guppies and sharks), and maybe most famously seahorses where males become pregnant.
Fish spawning can be divided into guarders, non-guarders, and bearers which have internal development of the offspring. Fishes are typically quite far on the r-selected side of the continuum. Most typically, fishes are non-guarders and do not care for their eggs or offspring. Less commonly - but more conspicuously - fishes guard nests. We see this in bass and Bluegill in the spring. Male only parental care is quite unique in the animal world. Care in bass and Bluegill is rather short compared to many other animals, lasting maybe a couple of weeks until the offspring are a little less vulnerable but much less than in humans where parental care extends past sexual maturity.
If you remember nothing else from this post, a favorite quote related fish...
I don't drink water, fish (fornicate) in it. - W.C. Fields
Though it is not all that true for most fishes as they simply throw their gametes out there and let the water do the rest.